The present invention relates to an apparatus for inspecting a circuit pattern drawn on a photomask (reticle) used in manufacturing large scale integrated circuits (LSIs).
An apparatus is disclosed in an article entitled "A Reticle and Mask Automatic Defect Inspection Apparatus", the Journal "Denshi Zairyo" (Electronic Material), pp. 50 to 54, No. 6, 1982, issued in Japan, which compares measured pattern data obtained by optically measuring a circuit pattern of a photomask with design (original) pattern data used in drawing the circuit pattern on the photomask to detect photomask defects. The design pattern data is provided in the form of binary data for each pixel of the photomask.
In the conventional apparatus, the design data is converted to data of dots each having the same size as the measured pixel which depends upon the size of an image sensor element. A measured value from the image sensor element is given in the form of binary data and is compared with the design data in units of dots. In the apparatus, the measured point on the photomask measured by the image sensor element can only be expressed in units of dots. Intermediate measured values are rounded off to "1" or "0" for bi-level quantization. Therefore, the determination of a pattern defect is made only when a difference between the design data and the measured data exists with respect to 2 or 3 consecutive dots, allowing for measurement errors. Since the size of the measured pixel, depending on the size of an image sensor element, is the same as that of design pattern data, the pixel size of the design pattern data must be converted for each of photomasks which have different scaling factors. In order to improve the defect detection precision in the inspecting apparatus of this type, the size of the pixel to be measured must be decreased. In this case, in order to maintain the measuring speed even if the pixel size is decreased, the rate at which the photomask is scanned by the image sensor must be high, inversely proportional to the square of the pixel size, resulting in serious problems.